Ethylene-vinyl alcohol copolymers (hereinafter, may be abbreviated as “EVOH(s)”) are useful polymeric materials that are superior in barrier properties against various types of gases such as oxygen, oil resistance, antistatic properties, mechanical strength and the like; therefore, EVOHs are formed into films, sheets, and the like, and widely used as various types of packaging materials, containers, etc. In particular, laminates constituted with an EVOH layer and other thermoplastic resin layer are known to be useful as packaging materials for boiling sterilization or retort sterilization of foods.
However, when the laminates are subjected to generally employed boiling or retorting sterilization using hot water, water penetrates into the EVOH layer during the processing, leading to deterioration of mechanical properties of the EVOH layer. As a method for an improvement of the mechanical properties, blending the EVOH with a polyamide (hereinafter, may be abbreviated as “PA”) having superior hot water resistance has been conventionally employed (hereinafter, suitability for such boiling sterilization or retorting sterilization may be also referred to as “retort resistance”). In these days, as a method for further improving the retort resistance, the following methods have been developed: a method involving laminating a layer formed from a resin composition having the mass ratio of EVOH/PA of 55/45 or more and 97/3 or less as an outermost layer, and a layer formed from a thermoplastic resin having low moisture permeability as an inner layer (see Japanese Unexamined Patent Application, Publication No. H10-80981); a method involving incorporating a metal compound and/or a boric acid compound into an intermediate layer formed from a composition containing EVOH and a PA (see Japanese Unexamined Patent Application, Publication No. H4-131237); and a method involving forming an intermediate layer from a composition containing two types of EVOHs and PA (see Japanese Unexamined Patent Application, Publication No. H6-23924).
In the conventional resin compositions containing EVOH and PA, the retort resistance has been improved. However, a crosslinking reaction may proceed between a hydroxyl group or a terminal carboxyl group of the EVOH and an amide group, a terminal amino group or a terminal carboxyl group of the PA, leading to nonuniformity of a resin viscosity, and resulting in significant generation of burnt deposits within an extruder, a screw and a die during melt molding over a long time period, and the like.
Such burnt deposits generated within the extruder, the screw and the die may remain therein for a certain time period and thereafter contaminate formed articles during a continuous operation over a long time period. The burnt deposits thus contaminating the formed articles not only deteriorate an appearance, but also cause defects to be formed, resulting in deterioration of various mechanical properties. Typically, in order to prevent the burnt deposits from contaminating formed articles, it is necessary to periodically shut down the operation and to carry out disassembly for cleaning of a line of extrusion equipment. However, an increase of the frequency of this procedure leads to not only an increase in production costs, but also consumption of materials required for the shutdown and restarting as well as a loss of production time; therefore, improvements have been desired in light of both resources and costs.
As described in the foregoing, it is difficult to improve the retort resistance of the resin composition and additionally preclude a cost increase and the unfavorable appearance due to burnt deposits formed in an operation over a long time period based on the conventional techniques. Additionally, the odor in molding must be taken into consideration from an environmental viewpoint.